421 4.7.0 Temporarily rate limited: low reputation of the sending IP

Gmail is not refusing your mail for good. It is slowing it down because it does not trust the IP address (or domain) it comes from.

Why it happens

A new IP address or domain with no sending history.
A sudden jump in volume.
Recipients marked earlier mail as spam, or many addresses bounced.
A shared IP used by other senders with a bad record.

What to do

Let your mail system retry. Do not push the same mail harder.
Send less per hour and raise the volume slowly over days.
Send first to the people who open your mail, and drop addresses that bounce. Check the list before the next send.
Make sure SPF and DKIM pass: use the SPF, DMARC and DKIM checker.

Don't confuse it with

Gmail uses 421 4.7.0 for several things. If the text talks about a "PTR record", the fix is reverse DNS. If it says "TLS required", the recipient only accepts encrypted connections.

Questions

Will the email still arrive?

Usually, later. If Gmail keeps deferring it for days, your mail system gives up and reports a bounce.

Are the addresses bad?

No.

Got a different bounce?

Paste the whole message into the bounce code lookup. It reads the code and the words and tells you whose move it is.

421 4.7.0: Temporarily rate limited: low reputation of the sending IP

Fix your sending setup Blocked or slowed down as possible spam